The present invention relates to a tubular shaped piezo-electric sensor, with a high sensitivity, adapted to form continuous hydrophones of very great length while resisting appreciable variations of the hydrostatic pressure.
Tubular piezo-electric sensors of the above-mentioned type may comprise, for example, a tube made from a material having piezo-electric properties such as, for example, a polymer of the polyvinylidene fluoride type (PVF 2) with an electrode being associated on each of the faces thereof.
The inner electrode is generally formed of a metal film deposited on the inner face of the piezo-electric tube or is associated with a central core. When the central core is made from a dielectric material, the electrode may be formed by metallization of its outer surface or else by a spirally wound wire. The central core may be further made from a conducting material, for example polymer made conductive by the inclusion of metal particles, or else in the form of a rectilinear metal wire.
Sensors of the aforementioned type are disclosed in, for example, French Patent Nos. 2 145 099, the French Patent applications Nos. 87/09262 and 81/22452, the latter being published under the Nos. 2 517 155, the U.S. Pat. No. 4 568 851 or the PCT Patent application No. 86/00757.
In French Patent No. 2 145 099, a tubular piezo-electric sensor is formed of a central core around which is wound in a spiral at least one piezo-electric sensitive element. The piezo-electric sensitive element is formed of a strip of flexible piezo-electric material coated with a metal film on each of its faces. This sensor structure makes it possible to construct hydrophones of very great length which are insensitive to accelerations and flexions, and which are suitable for numerous underwater applications, particularly in geophysics. The use of sensitive elements in the form of relatively thick piezo-electric films offers undeniable economic advantages.
In an article published in the J. ACOUS. SOC. AM 68 (4), October 1980, pp 1025-1029 a cylindrical piezo-electric hydrophone is described which includes a tubular support coated with a piezo-electric film on its inner surface and/or on its outer surface and cosed at each end by a rigid cap, with an inner volume of the tubular support being filled with a gas. Such a structure has increased sensitivity because the gas-filled tubular support may be deformed under the action of the radially exerted forces and thus provides mechanical amplification. But in the field of marine applications, where the depth of use is very variable, it would reveal itself to be unfavorable. Since the inner volume of the tubular support is compressible, the tubular support would not resist very well too high a rise of the hydrostatic pressure and there would be a danger of sealing losses or breakage of the sensitive element. It should also be noted that the construction of a piezo-electric sensor with a sensitive film coating the inner surface of the tubular support is very difficult in practice if its length is very great.
In accordance with the present invention, a sensor having both a very high sensitivity and a capacity to withstand high hydrostatic pressure variations is provided which comprises at least one sensitive element made from a material having piezo-electric prperties and associated with electrodes, with the sensitive element being disposed on a tubular support.
Advantageously, according to the present invention, an inside of the tubular support contains at least one substance whose acoustic impedance is very different from that of the material forming said tubular support, so as to form an acoustic screen for the acoustic waves passing through the tubular support, and means are provided for maintaining a pressure inside the tubular support substantially equal to the static pressure prevailing outside the piezo-electric sensor.
The inside of the tubular support of the present invention contains, for example, at least one substance impregnated with a liquid. The liquid impregnated substance may advantageously be formed of a fibrous material such as, for example, glass fiber or else from a porous material.
Depending on the embodiment of the present invention, the tubular support having at least one sensitive element and the screen forming substance is disposed inside a sealed sheath transparent to the acoustic waves and filled with liquid, with respective, ends of the tubular support comprising apertures for permitting the liquid filling the sealed sheath to penetrate therein.
By virtus of use of a substance effective in very considerably attenuating the acoustic waves inside the tubular support as proposed by the present invention, the sensitivity of the piezo-electric sensor within large proportions increases. The combination of this substance with means for providing a static balance between the pressure inside the tubular support and the outside pressure, and, more particularly, a liquid impregnated fibrous or porous material substantially at the same pressure as the external medium, makes it possible to construct a very sensitive and very resistant piezo-electric sensor. Such a sensor lends itself then very well to measurements in water at very varying depths.
Since, according to the present invention, each sensitive element may be wound in a spiral about the tubular support, and since the substance forming the inner acoustic screen is easy to position, sensors of great length may be constructed at relatively low cost. Sensors of this type disposed inside a very long sheath filled with liquid make it possible, for example, to construct receiving devices for seismic prospection, underwater listening or other geophysical or oceanographic operations.